I perceive the monsters are divided into more roles than just the “classes” listed in the DMG/monster manual. I’ve classified the heroic-level monsters and listed the actual roles I see (excluding comment on Minion and Solo monsters). It was taking too long to add commentary, so I decided to go ahead and publish what I wrote so far.
For some of the roles I listed some styles within the role.
Brute: A hand-to-hand monster which specializes in just dealing and soaking up damage. This is the simplest and easiest type of monster to run, and is very popular.
A classic-style brute has low accuracy and armor class, but high damage and hit points.
A soldier-style brute has high armor class instead.
A mobile brute has an easier time moving around than a regular brute, giving it more tactical flexibility. But it still doesn't really need to move around, otherwise it would be a skirmisher.
A charging brute really enjoys charging when it has the opportunity. But usually it is tied up by the opponents and can't do so.
Soldier (Defender): A hand-to-hand monster with abilities that hamper or distract the characters so that they can't get past them and attack other monsters. This usually involves marking or slowing the characters. Monster usually have very weak powers in this department, so in practice most soldiers feel very similar to brutes.
A classic marking soldier actually does mark the opponent, making it hard for him to attack any other character.
A distracting soldier doesn’t explicitly mark draw attacks to itself, it just has some powers which tend to slow down the players.
Grappler: A hand-to-hand monster which concentrates on one character, usually immobilizing the character, then neutralizing or destroying that character. A grappler is similar to a super-soldier. The difference is that the victim of a successful grappler tends to be in serious trouble and in need of rescue, while the opponent of a successful soldier is forced to fight the soldier but not otherwise in any particular danger.
One could easily argue that the grappler is the true soldier role and the role I listed as soldier is just half way in between.
A devouring grappler threatens the target with massive damage if he doesn’t escape.
A lockdown grappler doesn’t cause extra damage, it just immobilizes the target, making some powers hard to use and stripping the target of much of its tactical flexibility.
Skirmisher: A monster which constantly moves around a lot. A skirmisher has powers which prevent itfrom being effective by standing still and fighting a single opponent. In theory, a skirmisher could make the battle more exciting by forcing it to be more fluid and move around the map. I haven't yet had the opportunity to try a lot of true skirmishers and see if this actually works in practice.
A full skirmisher fights in hand-to-hand but has to move around a lot - creating an unusual dynamic in the fight. Often, a melee skirmisher ends the turn at a distance from his target.
An optional skirmisher is similar to a full skirmisher, but is fully capable of standing still and tying down opponents if it wants to. It just doesn't suffer any penalty from moving great distances during hand-to-hand combat, so it often does so to gain maximum tactical advantage.
A ranged skirmisher is like artillery, but is forced to constantly keep moving. Since even normal artillery are free to move while fighting, I haven't so far found this to be an incredibly interesting dynamic. Mostly it appears to mean that the monster is even more hosed than usual when attacked by a fighter.
A charging skirmisher likes to charge so much that his powers make it desirable to move off then charge, even when the monster is engaged by opponents capable of using opportunity attacks.
Controller: A monster, usually ranged, which concentrates on doing things to help other monsters instead of causing damage itself. Controllers cause lots of confusion and problems to the players, but really like having powerful monsters near them to cause damage once the players have been weakened.
A Leader is not a true role. A leader assists his allies just by fighting in the normal way. Unlike a controller, he does not have to “try” to support his allies.
Artillery: A monster which specializes in ranged combat.
A standard artillery fires normal ranged attacks at the players. They are free to either stand still or move around.
An area effect artillery uses attacks with area effects, encouraging the players to spread out.
A sneaky artillery tries to hide and get concealment to protect itself while it fires ranged attacks at the enemy.
Flanker: A hand-to-hand monster which gains a major bonus by earning combat advantage, and thus tries very hard to flank the players. A flanker needs to work with other monsters to be effective. Although a flanker doesn't necessarily need to move around a whole lot in combat once he is in position, he needs to move enough to maintain his flanking position. Depending on the structure of the battle, it may be useful for a flanker to spend an entire turn trying to get to the other side of the opponents. This is likely to make flankers move around more than brutes, but probably not as much as true skirmishers since flankers have less flexibility about where to move - they have to moving to flanking position on the target they want to attack.
Dragon: This is the best name I came up with for a damage-dealing monster than cannot fight just in melee or just at ranged, but must switch between different ranges, possibly including close area effect attacks. Many controllers also have differing ranges, but the difference is that a dragon does damage all by itself and doesn’t have to concentrate on helping allies.
The following roles are very rare:
Lurker: A monster which makes a big attack, then retreats to hide for one or more rounds until it is ready to make another big attack. This type of monster is described in the DMG, but doesn't really seem to exist in the monster manual, except for the Imp. The gargoyle is a sort of lurker variant – it can leave the battle to heal very effectively, but this won’t work in conjunction with other creatures because the battle will be over before it returns.
Versatile: This monster can choose between more than one role – generally picking between brute and artillery. Unlike a Dragon, a Versatile monster has no need to switch roles if it doesn’t want to – if it needs to, it can happily spent the entire battle as either a melee or a ranged fighter.
Hazard: This monster has a huge aura or close burst attack that is just as dangerous to allies as to enemies. This will presumably dominate the entire tactics of the combat.
Other: The Trap Haunt is just interesting. Beats me how to classify it.